Optical lithography is the patterning technology commonly used in making integrated circuits. The technology includes projection step-and-repeat exposure and alignment systems that produce high resolution images for producing micron and submicron size features. Since the tolerance of these optical systems is rather small, planarity of the integrated circuit device during the various stages of fabrication is important. Another important consideration, one to which this invention is directed, is the effects of the well known standing-wave phenomenon. This phenomenon effectively modulates the energy coupled into the layer of resist as a function of resist thickness and reflectance of the surface being coated. This effect is due primarily to multiple reflections from the aluminum metalization commonly used for interconnect wiring. The higher the reflectivity, the more pronounced the standing-wave effect. In device fabrication, the practical resolution of a projection aligner is limited by surface topology and reflectance of the substrate. On a planar and low reflectance substrate, micron and submicron features can be resolved by the current projection technology. On a nonplanar and highly reflective substrate, the standing-wave effect becomes severe and causes narrowing of the resist lines near the topographic steps. The minimum feature size degrades to approximately 1.5 .mu.m. This standing-wave effect which is caused by multiple reflections from the substrate can be reduced by using a low reflectance substrate. The most desirable reflectance is between 20% and 40%. In this range, the multiple reflections is much reduced, while there is enough reflective light to allow accurate alignment and focusing of the projection aligners.
Interconnect wiring or metalization for integrated circuit devices, is typically aluminum, aluminum alloy, refractory metal and metal silicide, or doped polycrystalline silicon. There is shown in FIG. 1 a typical prior art arrangement, wherein an aluminum conductor 10 is disposed on a surface 12 of a semiconductor device 14. The surface 12 is typically a surface of a layer of insulating material such as silicon oxide, or areas of contact with silicon or other metals. The upper most surface 16, of the aluminum conductor 10 is usually highly reflective thereby giving rise to the standing-wave phenomenon. Another prior art structure is shown in FIG. 2 which differs from that shown in FIG. 1 only by the addition of a cap layer 18 of refractory metal on the surface 16. The purpose of this cap layer 18 is to increase electromigration resistance and reduce roughness. Where the cap layer 18 has a thickness of about 1000 angstroms, the formation of hillocks is substantially suppressed. The combined reflectance of the surface 16 of the aluminum conductor 10 and the surface 20 of the cap layer 18 is reduced somewhat from that of the surface 16 if the cap layer 18 was not present. This reflectance, depending on the thickness and material of the cap layer 18, can be no less than 40%.
What is needed is a low reflectance conductor suitable for integrated circuit device applications which does not greatly increase processing complexity.